In automotive engineering electromechanical drive systems are used, for example in window regulators, in seat adjusters, in sunroof adjusters and for other accessory drive systems. They consist of an electric motor that is arranged in the cross-arm of a bridge circuit (also referred to as H-circuit). The armature current of the electric motor is specified using control elements of the bridge circuit. The control elements are controlled by control signals that provide a control circuit. The control elements are frequently formed using a relay for reasons of simplicity and costs. However, the bridge circuit can consist completely or partly of semiconductor switches. The control circuit usually contains a microprocessor or a microcontroller.
In a motor vehicle, an electromechanical drive is exposed to rough operating conditions. Pulse-shaped stress increases (load dump) can occur in the vehicle electrical system that can lead to a malfunction of the control. If the stress increase occurs during a control process of the relay, this can lead to a thermal overload of the mechanical contacts of the relay. If a normally open contact of a relay “hangs” on the positive counter contact, the motor could operate against a mechanical limitation and could be thermally overloaded. If there is no protective device and the error is unnoticed, this overheated motor can cause a fire. This poses a considerable safety hazard in a motor vehicle.
In order to prevent a thermal overload of the motor, the use, for example, of a temperature sensor to record the winding temperature of the motor, is known from prior art. A temperature sensor can fail and is also expensive. The other alternative is to use a monitoring circuit to monitor the operating state of the bridge circuit and thus the potential on the motor connecting terminals.
What poses to be a problem here is the expenditure that results from the fact that every returned state signal allocates an input signal connection to the control circuit. If the control circuit is designed, for example as a microcontroller, then the type, thus the overall size and also the price of the microcontroller are predetermined using the number of the returned signals. If the control circuit is supposed to be designed in a miniaturized form as a mechatronic module, it is disadvantageous to use a microcontroller having many I/O pins.